Genome-wide Identification of MAPK- MAPKK Gene Families in Juglans regia and their Expression Profile under Chilling Stress

Walnut is a temperate deciduous tree and late-spring frost often causes serious damage to its growth and development. MAPK- MAPKK are widespread in eukaryotes and play an important role in the stress response by regulating the expression of stress-related genes in response to environmental changes. In this study, we screened for the MAPK-MAPKK gene family in the walnut genome database. We found 16 MAPKs, 8 MAPKKs members. Subsequently, we analyzed the structure of each member and performed a phylogenetic analysis. The two families of MAPK-MAPKK were divided into four subgroups, and similar gene structures and conserved motifs were observed in each subgroup. Analysis of the promoter sequences of each member showed that they contained abundant hormone and stress response elements. Transcriptome sequencing showed that the expression of each family member was specific in different tissues and induced by chilling stress. Further, the transcription level of each family member under chilling stress showed a significant correlation in the correlation analysis, and JrMAPK2 , JrMAPK3 , JrMAPK10 , and JrMAPK15 were directly related to COR1 , COR4 , and COR7 by qRT-PCR. Gene structure, genome evolution, and expression analysis uncovered a relationship between the MAPK-MAPKK cascade in walnut, and that JrMAPK2 , JrMAPK3 , JrMAPK10 , and JrMAPK15 genes might be pivotal for the walnut chilling response pathway. In this study, we systematically describe the MAPK, MAPKK families in walnut for the first time, and understanding the cascade network of MAPK, MAPKK in plants will help lay a foundation for elucidating their function in the walnut chilling stress response.